1. Field of the Invention
The present invention relates to a commutator and an electric rotary device, e.g., a motor, an electric generator, having the commutator.
2. Description of Background Art
A conventional commutator of an electric rotary device, e.g., a motor, an electric generator, is shown in FIG. 5. Commutating piece 12 of the commutator 10 are made of a copper alloy, and they are held on a core 14, which is made of, for example, phenolic resin.
The commutating pieces 12 of a micro motor are often made of a clad material, which is made by adhering two metals, e.g., a copper alloy and a gold-silver alloy.
Another conventional commutator is shown in FIG. 6. In the commutator 10, sliding members 16 are respectively fixed on outer faces of commutating pieces 12, which are made of a copper alloy, and they are held by a plastic core 14. Each of the sliding members 16 is manufactured by the steps of: mixing graphite with a binder, e.g., pitch, plastic powders; molding the mixture with applying pressure; and baking the molded body.
In the conventional commutator 10 having metallic commutating pieces 12 on which a brush slides, oxide films are formed on sliding faces of the commutating pieces 12, on which the brush slides, by oxygen in the air. Since the oxide films is an insulating material, an electric current passes between the brush and the commutating pieces by breaking the oxide films.
Micro projections and cavities are formed in surfaces of the commutating pieces 12, so the brush point-contacts the commutating pieces 12. Namely, contact area between the both is very small. Therefore, electric resistance between the brush and the commutating pieces 12 are increased, the brush and the commutating pieces 12 are overheated due to small contact area therebetween, and adhesive wear, which is caused by adhering the brush to the commutating pieces 12 and peeling the adhered parts, occurs in the brush and the commutating pieces 12.
In the case of highly overheating the brush and the commutating pieces or generating an arc therebetween, both members are worn by metal transfer, dispersion of a melted metal.
In another case, abrasive wear, which is caused by forming hard substances, e.g., oxides, on one or both of the sliding faces of the brush and the commutator and grinding soft parts with the hard substances, occurs when the metal transfer progresses. Further, metal powders formed by the hard substances accelerate the abrasive wear.
If the adhesive wear or the abrasive wear is highly progressed, a span of life of a commutating mechanism, which includes the brush and the commutator, is greatly shortened. However, the oxide films formed on the surfaces of the commutating pieces can be broken by wear, so that an electric current can pass between the brush and the commutating pieces. Therefore, a little wear of the commutating piece is required for operating the electric rotary device.
In the case of the micro motor, current intensity passing through the motor is so small that enough energy for breaking oxide films cannot be gained. Thus, expensive gold is included in the sliding faces of a commutator, on which a brush slides, so as not to form oxide films thereon.
In the commutator 10 shown in FIG. 6, the sliding members 16 include graphite so as to solve the problem of forming oxide films. However, crystals of graphite are unstable, so oxidization wear occurs in the sliding faces of the sliding members 16 when graphite is heated at 400° C. or higher in the air. By the oxidization wear, graphite will be consumed as CO or CO2. Therefore, it is improper to use the commutator 10 in the air.
In the case of electric rotary devices using the conventional metallic commutating pieces 12, it is difficult to gain enough sliding between the parts.
On the other hand, in the case of electric rotary devices using the conventional sliding members 16 including graphite, graphite provides enough sliding.
However, graphite employed in the conventional commutators has a layered crystal structure. Electric conductivity in the direction along faces of crystal layers is much greater than that in the direction perpendicular to the faces of the crystal layers. By the anisotropy, contact resistance between graphites and between graphite and metal highly vary according to contact directions therebetween. For example, in FIG. 7, an electric current having enough intensity flows in the direction along faces of the graphite 18; the current passes along an arrow. Therefore, efficient must be low. Note that, symbols 17 stand for copper powders, and symbol 19 stands for a brush.
Further, many projecting parts and voids exist in the commutator, so the area of contacting the brush 19 must be small; the sliding of the commutator must be bad in spite of including graphite. Due to the bad sliding, the exhaustion (the abrasive wear, the arc wear or the oxidization wear) is apt to be occurred, so that the span of life of the commutator must be short.